PV Elite

Analyzing openings in Skirts of Vertical Vessels

Vertical vessels that are Skirt supported typically have access openings in the skirt. These openings need to be analyzed specially when they are of reasonable size, are on tall towers or are under sizable Wind/Seismic loads. This article explains skirt openings input in PV Elite.

Entering the Data

Users sometimes ask: 'How must we enter the information'. Well, first indicate that the Skirt element has the opening(s). Check the box to indicate so, as shown in the screen shot below:

This will open the following dialog. Let us consider the various features of the data entry table. It looks like this:

image hole entry table

Let us consider the various components of the table.

Number of Skirt Openings (0-10)

From the drop down list box, choose the number of holes in the skirt. There can be up to 10 holes.

Width, Height, Center Spacing, Frame Thickness, Frame Width

Here are the dimensions:

image hole geometry

Layout Angle

The layout angle is the location (in plan view) around the skirt:

image layout angle

Pitch and Ligament

In the skirt input two very important dimensions are mentioned, and please make note of them. They are the pitch and the ligament. Here are those dimensions:

image ligament and pitch

The pitch is the center to center distance between adjacent holes. The ligament distance is very important. If the ligament is too small there is a possibility of collapse and buckling, which is a very dangerous situation. EN 13445, the code chosen as the basis for the analysis is silent on this issue. This is because EN 13445 only gives consideration to one hole, not the multiple holes that may be analyzed by PV Elite.

The output processor (results) gives a recommended minimum ligament distance, but the user must evaluate whether this meets project requirements. The following image shows a sample of the results.

Skirt Opening Analysis Results

Vessels with Multiple Skirt Elements

Note: These next notes only apply to PV Elite after the March 2007 build

Vessels with multiple Skirts raise some interesting questions.

  • What if the skirt is made from two separate pieces?

  • What happens if the access opening or hole spans the two skirts as shown in Figure 1?

Analyzing access openings and other holes in skirts can be accomplished by observing a few precautionary measures. Complications can arise when there is more than one skirt supporting the vessel. Figure 1 illustrates this:

                                                            Figure 1

Before we discuss the hole that spans the bottom and top skirts, let us first consider some very important points:
  1. The two skirts may be different thicknesses
  2. The two skirts may be different materials
  3. The two skirts may have different taper (conical) angles.
Therefore, if we have an access opening that spans both skirts, the hole cannot be considered as ONE hole in 'one skirt'. The analysis would be very difficult. Does this mean that a hole cannot be analyzed that spans two skirts? Happily, there is a simple way of analyzing the single hole that spans two skirts this way.

Look at figure 2:

split skirts
                                                                  Figure 2

Note, we have split the skirts so each skirt is dealt with separately. This is the proper way to proceed:
  1. In the bottom skirt, enter the hole geometry. The height of the center of the hole is located near the top of the skirt. Don't worry that the top half of the hole is outside the top node of the skirt. PV Elite can handle this without any problems. You will just get a message that the hole breaks out of the top of the skirt. Otherwise, the analysis is correct.

  2. In the top skirt, again, enter the hole geometry. In this case the height of the hole is zero (or near zero). The analysis will be performed, but this time you get a message that the skirt breaks out of the bottom of the skirt. PV Elite can handle this situation.
Let is look at some more issues that may cause confusion.

PV Elite is designed such that the geometry allows entry of obround holes. For example, with an obround hole, the sentry box type geometry as shown in figure 3 can be utilized:

image sentry box opening

                                                                Figure 3

The bottom of the obround opening breaks the bottom of the skirt, but PV Elite merely issues a note to that effect, but performs the analysis without any problems.

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